The present invention relates to an improved cavity filter that can be readily manufactured and worked to a high degree of precision. More particularly, the invention relates to a particular design of the cavity bottom of such a filter.
When constructing and manufacturing high-frequency filters for radio base stations, the filters are built-up to form so-called cavity filters that consist of a plurality of cavities, either with a separate centre conductor in each cavity or with more than one centre conductor per cavity. These filters are used, for instance, in base stations for GSM-based mobile telephony at the frequencies of 900 MHz and 1800/1900 MHz.
Each cavity and its centre conductor/conductors functions as an electric oscillating circuit that can be represented by a parallel oscillation circuit having an inductive part L and a capacitive part C when the filter is turned to a quarter wavelength of the received signal. The inductive part is determined essentially by the length of the centre conductor, while the capacitive part is determined essentially by the diameter of the centre conductor and its distance from the cavity side walls and a triming plate provided on the cavity. The filter cavities are disposed in juxtaposed relationship, so that the electromagnetic field generated by the oscillations in one cavity will induce an electric current in an adjacent cavity and therewith also generate oscillations therein. Adaptation of the filter properties is most often achieved through the medium of openings in the walls between two adjacent cavities, so as to enable the coupling factor to be trimmed. The width of the openings between adjacent cavities or the height of the walls therebetween may be variable so as to enable a correct coupling factor to be obtained. However, a cavity filter may also be constructed without walls between mutually sequential centre conductors. In such a filter construction, trimming of the coupling factor between centre conductors can be effected by changing the distance therebetween.
It will be evident from the aforegoing that one important aspect of the function of the cavity filter is that the centre conductor is disposed on a very flat cavity bottom. Since the depth of the cavity influences the function of the cavity, it is essential that the filter body can be manufactured to precise measurements with respect to the distance between the cavity bottom and the upper edge of respective walls, so as to achieve good parallelity between said bottom and said upper edges.
An example of an earlier known cavity bottom construction is shown in FIG. 1, which also shows a known centre conductor. The Figure illustrates in cross-section a centre conductor 13 mounted in a cavity that comprises a cavity-defining body 11 and a cavity bottom 12. In this known construction, the bottom 12 is formed in the same working step as the manufacture of the cavity in general. This method of manufacture, however, means that the cavity bottom will not obtain the surface finish, planarity or flatness, that is required in order of the centre conductor to fulfil its function in the best way.
According to this known solution, the centre conductor 13 is hollow so that it can be secured to the cavity bottom 12 by means of a screw 14 passed through openings in the bottom surface of the centre conductor and the bottom of said cavity. The electric junction between the centre conductor and the cavity bottom is comprised of the bottom surface that lies around the screw, which bottom surface may be deficient with respect to surface finish and flatness in the case of this known construction.
It is thus desirable to obtain a cavity filter that has the aforedescribed advantageous properties and that can be manufactured readily despite the high precision requirements involved.
The present invention relates to an improved cavity filter construction.
A first object of the invention is to provide a cavity filter that can be readily manufactured despite high precision requirements.
Another object of the present invention is to provide a cavity filter where only certain parts of the cavity and the upper edge of the side walls need be precision worked so as to obtain a very flat surface on which the centre conductors are secured while achieving, at the same time, precise measurements and parallelity between the bottom surface and the upper edge of said cavity.
These objects are achieved with an inventive cavity filter that includes a plurality of centre conductors which are disposed on a flat groove or a flat elevation that is preferably elevated slightly above the bottom of the cavity or on a cylindrical portion that forms an integral part of the cavity filter. The cavity filter need not include any intermediate walls.
A first advantage afforded by the inventive cavity filter is that it enables manufacture to be simplified. Instead of accurately working, e.g. machining or treating, the whole of the cavity bottom in order to obtain a flat bottom surface, it is only necessary to machine the aforesaid groove or elevation or the upper surface of said cylindrical portion in order to obtain the requisite degree of flatness. This working can be effected in one single manufacturing stage, by excluding the walls between the cavities. The obtained flat surface provides a reference surface in the continued working of the cavity body, so as to obtain an exact measurement and good parallelity between the centre conductor contact surface and the upper edge of the cavity body.
Another advantage afforded by the inventive cavity filter is that trimming of the coupling factor between mutually adjacent centre conductors is simplified. The walls between two centre conductors are not required for trimming purposes, and all that is necessary is to change their distance from the machined groove or elevation.